This invention relates to cathodes for electrolytic cells for the production of aluminum, and specifically to the preparation of cathode tiles. The cathode tiles are prepared of electroconductive materials, and have a surface that is aluminum wettable and contains refractory hard materials.
Aluminum is conventionally manufactured by an electrolytic reduction process conducted in Hall-Heroult cells, wherein alumina is dissolved in molten cryolite and electrolyzed at temperatures of 900-1000.degree. C. These cells typically comprise a steel shell with an insulating lining of suitable refractory materials, which in turn is provided with a lining of carbon which contacts the molten bath, aluminum, and/or ledge. One or more anodes, usually made of carbon, are inserted into the molten cryolite and connected to a positive pole of a direct current source. The negative pole of the direct current source is connected to the carbon lining in the bottom of the cell. Molten aluminum resulting from the electrolytic reduction reaction is deposited on the carbon bottom of the cell in a molten pool or pad, which acts as a liquid metal cathode. Part of this pool of liquid is removed from time to time and collected as the product of the electrolysis process.
In the construction of most commercial cells, the carbon lining that forms the bottom of the cathode is conventionally built from an array of prebaked carbon blocks covering the portion of the cell to be lined, and then the carbon blocks are joined into a solid continuous assembly by ramming the slots between blocks with a mixture typically of calcined anthracite, modified coal tar pitch, and the like. This structure is then heated in the process of cell start-up. Life span of such constructed carbon linings in different plants averages three to eight years, but under adverse conditions may be considerably shorter. Deterioration occurs due to penetration of electrolyte components and liquid aluminum into the structure of the carbon blocks, ramming mix, and refractory materials, causing swelling and cracking. Aluminum metal penetration causes alloying away of steel current collector bars embedded in the cell bottom, which contaminates the aluminum pad and may lead to cell tap-out.
Other problems include accumulation of undissolved bath and alumina which are carried from the cryolite bath, ledge, and ore cover, to the cathode, creating sludge or muck. The presence of this sludge or muck under the aluminum pad creates areas on the cell bottom which disrupt electrical current distribution, resulting in excessive pad turbulence and disturbances through magnetic forces, hence reducing cell current efficiency.
A further drawback of the carbon cathode lining is its non-wettability by molten aluminum, which necessitates a deeper pad of aluminum, to ensure effective molten aluminum contact to the carbon lining or surface. These deep aluminum pads are subject to magnetic and electrical effects, such as standing waves, which increase the possibility of electrical shorting. To lessen this possibility, greater anode-to-cathode distances (ACD) are employed, resulting in additional voltage losses.
To reduce ACD and associated voltage drop, cathode materials using Refractory Hard Material (RHM), such as TiB.sub.2, have been employed. TiB.sub.2 is highly conductive and is wetted by liquid aluminum. This wettability property enables a thin film of molten aluminum to be deposited directly on the cathode structure made of RHM, and eliminates the need for a pad of metal, since contact with the underlying cathode structure is assured.
The use of titanium diboride current-conducting elements in electrolytic cells for the production of aluminum is described in the following exemplary U.S. Pat. Nos.: 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093, and 3,400,061. Despite the rather extensive effort expended in the past, as indicated by these and other patents, and the potential advantages of the use of titanium diboride as a current-conducting element, such compositions do not appear to have been commercially adopted on any significant scale by the aluminum industry. Lack of acceptance of TiB.sub.2 or RHM current-conducting elements of the prior art is related to their lack of stability in service in electrolytic reduction cells. It has been reported that such current-conducting elements fail after relatively short periods in service. Such failure has been associated with the penetration of the self-bonded RHM structure by the electrolyte, and/or aluminum, thereby causing critical weakening with consequent cracking and failure. It is well known that liquid phases penetrating the grain boundaries of solids can have undesirable effects. For example, RHM tiles wherein oxygen impurities tend to segregate along grain boundaries are susceptible to rapid attack by aluminum metal and/or cryolite bath. Prior art techniques to combat TiB.sub.2 tile disintegration in aluminum cells have been to use highly refined TiB.sub.2 powder to make the tile, containing less than 50 ppm oxygen at 3 or 4 times the cost of commercially pure TiB.sub.2 powder containing about 3000 ppm oxygen. Moreover, fabrication further increases the cost of such tiles substantially. However, no cell utilizing TiB.sub.2 tiles is known to have operated successfully for extended periods without loss of adhesion of the tiles to the cathode, or disintegration of the tiles. Other reasons proposed for failure of RHM tiles and coatings have been the solubility of the composition in molten aluminum or molten flux, or the lack of mechanical strength and resistance to thermal shock.
Additionally, different types of TiB.sub.2 coating materials, applied to carbon substrates, have failed due to differential thermal expansion between the titanium diboride material and the carbon cathode block. To our knowledge no prior RHM-containing materials have been successfully operated as a commercially employed cathode substrate because of thermal expansion mismatch, bonding problems, etc.
For example, U.S. Pat. No. 3,400,061, of Lewis et al, assigned to Kaiser Aluminum, teaches a cell construction with a drained and wetted cathode, wherein the Refractory Hard Material cathode surface consists of a mixture of Refractory Hard Material, at least 5 percent carbon, and generally 10 to 20 percent by weight pitch binder, baked at 900.degree. C. or more. According to the patent, such a composite cathode has a higher degree of dimensional stability than previously available. The composite cathode coating material of this reference may be rammed into place in the cell bottom. This technique has not been widely adopted, however, due to susceptibility to attack by the electrolytic bath, as taught by a later Kaiser Aluminum U.S. Pat. No. 4,093,524 of Payne.
Said U.S. Pat. No. 4,093,524, of Payne, claims an improved method of bonding titanium diboride, and other Refractory Hard Materials, to a conductive substrate such as graphite, or to silicon carbide. The cathode surface is made from titanium diboride tiles, 0.3 to 2.5 cm thick. However, the large differences in thermal expansion coefficients between such Refractory Hard Material tiles and carbon precludes the formation of a bond which will be effective both at room temperature and at operating temperatures of the cell. The bonding is accordingly formed in-situ at the interface between the Refractory Hard Material tile and the carbon by a reaction between aluminum and carbon to form aluminum carbide near the cell operating temperature. However, since the bond is not formed until high temperatures are reached, tiles are easily displaced during startup procedures. The bonding is accelerated by passing electrical current across the surface, resulting in a very thin aluminum carbide bond. However, aluminum and/or electrolyte attack upon the bond results if the tiles are installed too far apart, and if the plates are installed too close together, they bulge at operating temperature, resulting in rapid deterioration of the cell lining and in disturbance of cell operations. Accordingly, this concept has not been extensively utilized.
Holliday, in U.S. Pat. No. 3,661,736, claims a cheap and dimensionally stable composite cathode for a drained and wetted cell, comprising particles or chunks of arc-melted "RHM alloy" embedded in an electrically conductive matrix. The matrix consists of carbon or graphite and a powdered filler such as aluminum carbide, titanium carbide or titanium nitride. However, in operation of such a cell, electrolyte and/or aluminum attack grain boundaries in the chunks of arc-melted Refractory Hard Material alloy, as well as the large areas of carbon or graphite matrix, at the rate of about one centimeter per annum, leading to early destruction of the cathodic surface.
U.S. Pat. No. 4,308,114, of Das et al, discloses a contoured cathode surface comprised of Refractory Hard Material in a graphitic matrix. In this case, the Refractory Hard Material is composited with a pitch binder, and subjected to graphitization at 2350.degree. C., or above. Such cathodes are subject to early failure due to rapid ablation, and possible intercalation and erosion of the graphite matrix.
In addition to the above patents, a number of other references relate to the use of titanium diboride in tile form. Titanium diboride tiles of high purity and density have been tested, but they generally exhibit poor thermal shock resistance and are difficult to bond to carbon substrates employed in conventional cells. Mechanisms of de-bonding are believed to involve high stresses generated by the thermal expansion mismatch between the titanium diboride and carbon, as well as aluminum penetration along the interface between the tiles and the adhesive holding the tiles in place, due to wetting of the bottom surface of the tile by aluminum. In addition to debonding, disintegration of even high purity tiles may occur due to aluminum penetration of grain boundaries. These problems, coupled with the high cost of the titanium diboride tiles, have discouraged extensive commercial use of titanium diboride in conventional electrolytic cells, and limited its use in new cell design. It is a purpose of the present invention to overcome the deficiencies of past attempts to utilize Refractory Hard Materials as a surface material for carbon cathode blocks.
This invention discloses a process for the manufacture of a Refractory Hard Material composition that can be prepared and formed into plates or tiles, can be extruded into plates or tiles, with a simple processing technique and heat treatment procedure.
Another object of this invention is to prepare tiles having a stratified or layer construction, allowing the RHM to be concentrated in a layer that will form the surface of the tile that will face the anode.